Merge pull request #48 from glen3b/patch-1

Minor improvements to ClassLoader

ProtocolLib/src/main/java/com/comphenix/protocol/utility/ClassSource.java

@@ -1,5 +1,6 @@
 package com.comphenix.protocol.utility;
 
+import java.util.Collections;
 import java.util.Map;
 
 /**
@@ -40,6 +41,7 @@ public abstract class ClassSource {
 
 	/**
 	 * Construct a class source from a mapping of canonical names and the corresponding classes.
+	 * If the map is null, it will be interpreted as an empty map. If the map does not contain a Class with the specified name, or that string maps to NULL explicitly, a {@link ClassNotFoundException} will be thrown.
 	 * @param map - map of class names and classes.
 	 * @return The class source.
 	 */
@@ -47,11 +49,45 @@ public abstract class ClassSource {
 		return new ClassSource() {
 			@Override
 			public Class<?> loadClass(String canonicalName) throws ClassNotFoundException {
-				return map.get(canonicalName);
+				Class<?> loaded = map == null ? null : map.get(canonicalName);
+				if(loaded == null){
+					// Throw the appropriate exception if we can't load the class
+					throw new ClassNotFoundException("The specified class could not be found by this ClassLoader.");
+				}
+				return loaded;
 			}
 		};
 	}
 	
+	/**
+	 * @return A ClassLoader which will never successfully load a class.
+	 */
+	public static ClassSource empty(){
+		return fromMap(Collections.<String, Class<?>>emptyMap());
+	}
+
+	/**
+	 * Retrieve a class source that will attempt lookups in each of the given sources in the order they are in the array, and return the first value that is found.
+	 * If the sources array is null or composed of any null elements, an exception will be thrown.
+	 * @param sources - the class sources.
+	 * @return A new class source.
+	 */
+	public static ClassSource attemptLoadFrom(final ClassSource... sources) {
+		if(sources.length == 0){ // Throws NPE if sources is null, which is what we want
+			return ClassSource.empty();
+		}
+		
+		ClassSource source = null;
+		for(int i = 0; i < sources.length; i++){
+			if(sources[i] == null){
+				throw new IllegalArgumentException("Null values are not permitted as ClassSources.");
+			}
+			
+			source = source == null ? sources[i] : source.retry(sources[i]);
+		}
+		return source;
+	}
+
 	/**
 	 * Retrieve a class source that will retry failed lookups in the given source.
 	 * @param other - the other class source.
@@ -106,7 +142,7 @@ public abstract class ClassSource {
 	/**
 	 * Retrieve a class by name.
 	 * @param canonicalName - the full canonical name of the class.
-	 * @return The corresponding class 
+	 * @return The corresponding class. If the class is not found, NULL should <b>not</b> be returned, instead a {@code ClassNotFoundException} exception should be thrown.
 	 * @throws ClassNotFoundException If the class could not be found.
 	 */
 	public abstract Class<?> loadClass(String canonicalName) throws ClassNotFoundException;